Blizzard is adding 5 endgame bosses to Diablo 4 in Season 2, allowing for targeted farming of Uniques and Uber uniques.
The good news is the bosses will be available on both Eternal and Seasonal realms.
By endgame bosses, Blizzard likely refers to encounters like Uber Lilith or Varshan. They plan on adding 5 new ones to the game on October 17.
If you’re looking for more details about Season 2, you will find them in the official preview.